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The invention relates generally to oscillators and, more particularly, to oven-controlled or xe2x80x9covenizedxe2x80x9d quartz crystal oscillators or OCXOs.
Oscillators are used to generate frequencies for applications varying from relatively unsophisticated applications for wristwatches and the like, to such extremely sophisticated applications as timing systems for space navigational systems. Most commonly, quartz crystals composed of SiO2 are used in oscillators, although certain highly accurate frequency standards can be configured using an atomic reference source, such as cesium or rubidium.
Precision OCXOs, that are both stable and accurate, are highly desirable for use in many applications. Currently, for example, both stable and accurate OCXOs are sought for use in the design of base stations for cellular, PCS (personal communication system), and wireless local loop (WLL) systems that connect subscribers to a public switched telephone network. OCXOs can be used successfully, for instance, in the transmit and receive functions or in the clocks of CDMA (Code Division Multiple Access) base stations.
The stability of a crystal is rated according to the extent to which the crystal""s inherent instabilities can be compensated. Significant contributors to the assessment of stability are the degree to which the frequency of the crystal changes with temperature, and the degree to which the frequency changes over the long-term, i.e., the aging characteristic of the crystal. Typically, the accuracy per year of OXCO is on the order 1xc3x9710xe2x88x928, and desired stability over a wide range of environmental conditions is better than 1xc3x9710xe2x88x9210. These conditions include operating temperature, humidity, supply voltage variations, repeatability, frequency-setting ability, and frequency drift over long periods of time. (Frequency xe2x80x9cdriftxe2x80x9d is distinguishable from xe2x80x9caging,xe2x80x9d insofar as the aging characteristic of a crystal is defined with reference to internal changes in oscillator when external factors, such as the environment or the power supply voltage, are constant.)
In an OCXO, the crystal and associated components, the latter of which might also be sensitive to temperature, are enclosed in an oven with a stable temperature. The temperature is kept constant by adjusting the amount of power supplied to the oven whenever the ambient temperature in the oven begins to change. The oven temperature selected is one at which the slope of the frequency vs. temperature curve for the crystal is zero. The oven thus minimizes the degree to which the frequency of the oscillator will vary with variations in temperature.
The realization of an OCXO typically requires (1) a reference element (e.g., a quartz crystal); (2) associated circuitry for frequency generation or synthesis; (3) a frequency tuning element or elements; (4) a thermal control system for the oven; and (5) an output buffer amplifier so that the signal output of the OCXO can be utilized.
In order to achieve an OCXO with a desired accuracy and stability, the precision of the reference element has always been of great significance. Unfortunately, the requirement for a precise reference element has limited the yield of crystal production and has kept the cost of creating OCXOs high. This is because the precision of the frequency of a crystal is affected by a great number of factors in the manufacturing process, such as the thickness of the cut of the crystal wafer, the angle of the cut, and imperfections or scratches on the crystal. The oscillator circuitry sensitivity to the frequency of the reference element likewise has contributed to manufacturing obstacles to large-scale and cost-effective OCXO production. Typical frequency-tuning components, such as inductors, capacitors and varactor diodes, are sensitive to environmental conditions, such as temperature, and repeatability and tolerance drift of these components over time must be taken into account in a typical OCXO design. The thermal control system for the oven had to be capable of achieving very accurate temperature settings adjusted for the characteristics of the particular reference element used.
Accordingly, those concerned with the design and manufacture of OCXO""s have long recognized the need for an OCXO which can manufactured with good yields in fairly large quantities and for reasonable cost. The present invention fulfills this need.
Briefly, and in general terms, the present invention provides an oscillator and method for realizing an oscillator that is a precision oscillator with desirable accuracy and stability over a wide range of environmental conditions, even while using a reference element with a frequency that is not as precise as has been necessary in the past. According to the present invention, an accurate and stable (better than 1xc3x9710xe2x88x9210/day) OCXO can be implemented using a reference element cut from a quartz bar to a thickness corresponding to, for example, 5.0033 or 5.0049 MHz (as opposed to, for example to 5.0000 MHz), and which has been manufactured without an especially precise cutting angle as otherwise would be required to achieve precise operating temperature characteristics. The OCXO according to the invention also is not dependent on reactive components, such as capacitors and the like, to tune and set the desired output frequency. Thus, by eliminating the necessity for cost-driving features normally associated with the manufacture of a precise reference element, the present invention results in a very precise OCXO that is reproducible and relatively easy to manufacture at reasonable cost.
The design of the OCXO according to the invention provides a stable reference source that is not wholly dependent on the precision of the reference element. More particularly, and by way of example and not necessarily by way of limitation, the present invention provides an OCXO characterized by a voltage-controlled oscillator (VCXO) which is configured to provide a desired predetermined output frequency, for example, 15 MHz. The VCXO frequency also is used by a high resolution frequency synthesizer to generate a VCXO adjustment frequency which, when added to the VCXO output frequency, will correspond to the frequency of the reference element or a multiple thereof. The sum of the adjustment frequency and the VCXO output frequency is then mixed with the reference frequency, or a multiple thereof, to create a substantially zero-beat feedback signal that is introduced into a phase-locked loop including the VCXO. The feedback signal thus locks the VCXO to the desired frequency. The high resolution frequency synthesizer thus insures that the VCXO output frequency is maintained as stable as the frequency of the reference element, but the accuracy of the output of the VCXO nevertheless will not be dependent on the precision of the reference element.
In a presently preferred embodiment, the VCXO adjustment frequency, is generated from the VCXO frequency using a direct digital synthesizer or DDS. In a DDS, adding circuitry or a phase accumulator is used to accumulate phase at a rate dependent upon the value of the frequency selected. The phase value is used to address some type of read-only memory (ROM), which stores discrete values of the sine function. The digital output of the read-only memory is converted to a sine wave by a digital-to-analog (D/A) converter. The sine wave is then low-pass filtered to remove such elements as the clock frequency and glitches due to the D/A conversion. However, it should be appreciated by one skilled in the art that other techniques for high resolution synthesis of a VCXO adjustment frequency from the VCXO frequency are possible using any method known in the art.
In accordance with a preferred embodiment, the VCXO RF output frequency (e.g., 15 MHz) is first applied to a buffer amplifier, and the output signal from the buffer amplifier is applied simultaneously (1) to another amplifier which precedes the input of the DDS, and (2) to a first mixer. The DDS synthesizes a signal, which has been predetermined to serve as an adjustment frequency for the VCXO. A divider preferably is used to divide down the signal from the DDS (e.g., to 10 kHz). The output of the divider is input to a first mixer, together with the VCXO output frequency. The mixer sums the two signals that are input to it (e.g., 15 MHz+10 kHz=15.01 MHz). The result of the first mixer then is input into a second mixer. The other input to the second mixer is the reference element frequency (e.g., 5.0033 MHz). The second mixer functions to multiply the reference element frequency by three and then subtract the result from the result of the first mixer (e.g., (3)(5.0033)xe2x88x9215.01=15.0099xe2x88x9215.01=0.0001). This substantially zero-beat signal from the second mixer is used to lock the VCXO, via a phase-locked loop, to maintain the VCXO output frequency. Given that the frequency of the VCXO output frequency is adjustable via the feedback signal derived from the high resolution DDS, the precision of the frequency of the reference element is not as critical to the stability of the oscillator as it would otherwise have been in prior art OCXO designs.
In some preferred embodiments of the invention, a microprocessor may be provided to generate various control signals for, e.g., the DDS to accomplish fine and coarse adjustments of the adjustable VCXO output frequency signals. For example, the OCXO may include an analog-to-digital (A/D) converter for generating electronic frequency control (EFC) signals. These EFC signals may be provided to the DDS for adjustment of the adjustable VCXO output frequency signals. Alternatively, a digital interface port may be provided to permit digital system interface controls to the DDS for frequency adjustment.
In other preferred embodiments, a temperature sensor preferably may be included for generating temperature control signals or signals to adjust the synthesizer frequency to accommodate the effect of temperature variations on frequency. The oven structure preferably might one with a thermal gain greater than 100,000, such as can be accomplished by using a zero-temperature gradient outer oven surrounding an inner oven.
Hence, the present invention satisfies a long-existing need for an oscillator with high stability and accuracy, which can be manufactured at reasonable cost with high yields.
These and other objects and advantages of the invention will become apparent from the following, more detailed description, when taken in conjunction with the accompanying drawings.